This invention relates to fluid flow controlling and, more particularly, to systems and methods for controlling gas flow.
There are many industrial and other applications in which it is necessary to measure and control the flow rate of fluids, particularly gases. Typically, gas flow is measured and controlled using volumetric flow devices such as turbine meters, rotometers, thermal mass flow rate control devices, or sonic gas velocity orifices.
The need for precision control is particularly acute in the semiconductor industry. Computer chip manufacturing requires exact control of various process fluids and gases, including but not limited to hydrogen, silane, helium, nitrogen, oxygen and argon. The current xe2x80x9cstate of the artxe2x80x9d in the semiconductor industry utilizes a sophisticated gas delivery system, often referred to as a gas panel incorporating xe2x80x9cgas sticksxe2x80x9d, which includes a mass flow controller, a pressure transducer, a filter, control valves and a pressure regulator, all connected in series. The flow control portion of these systems have high initial and maintenance costs, require frequent calibration and service to avoid inaccuracies caused by electronic drift and span, and may result in inaccurate flow rates when very high or very low flow rates are required.
In situations in which repeatability is more important than absolute accuracy, precision calibrated orifices have been used to provide a constant calibrated gas flow relative to gas supply pressure; if multiple fixed flow rates are needed, a number of orifices may be connected in parallel with each other with a switching mechanism for selecting the appropriate orifice. However, the use of such orifices is normally limited to applications that require one or more constant, non-variable gas flows. Even in fixed flow applications where their use is otherwise satisfactory, such orifices require high gas velocities which cause excessive turbulence, erosion therefore and flow instability, and are subject to plugging.
Precision porous sintered metal flow restrictors, (e.g., of the type manufactured and sold by Mott Corporation, the assignee of the present application and which have hundreds of interconnected through-pores or passages arranged both in parallel and series with each other) are also used to provide a specified down-stream flow relative to the applied upstream pressure. Such flow restrictors are less susceptible to plugging, clogging and wear than are conventional orifices, operate at relatively low flow velocities, and provide a smooth and constant down-stream flow. Like orifices, however, their use has been limited to applications that require an essentially constant and non-variable flow.
There remains a need for a system that, like a thermal mass flow controller, is capable of precisely measuring and controlling fluid flow over a range of flow rates and pressures, but that is more accurate over a wide range of flow rates, is less expensive, and that requires significantly less calibration, servicing and maintenance and is less susceptible to electronic drift and span.
The present invention features a method and system for controlling the rate of fluid, and particularly gas, flow which uses pressure regulation rather than a control valve. A flow restrictor having known pressure drop-flow rate characteristics is provided in a passage through which the fluid flows, the pressure drop across the flow restrictor is determined, and the pressure drop of the fluid flowing through flow restrictor is adjusted so that the actual pressure drop across the flow restrictor will closely correspond to the pressure drop associated with a desired flow rate.
In preferred embodiments a pressure regulator adjusts the pressure of gas upstream or downstream of the flow restrictor based on the pressure drop across the flow restrictor and with reference to data defining the pressure drop-flow rate characteristics of the flow restrictor, so that the actual pressure drop will closely correspond to the pressure drop associated with a desired rate of gas flow.
In particularly preferred embodiments, the flow restrictor comprises a porous sintered metal element, an upstream pressure sensor determines and provides data indicative of the pressure of gas in the flow passage upstream of the flow restrictor, a downstream pressure sensor determines and provides data indicative the pressure of gas in the flow passage downstream of the flow restrictor, the data from the sensors is compared with data indicative of the desired rate of gas flow and the known data representing the pressure drop-flow rate characteristics of the flow restrictor, and the system controls a pressure regulator (and hence gas pressure) on the basis of the comparison.
Fluid flow controllers embodying the invention comprise a fluid flow passage in which such a flow restrictor is positioned, and pressure sensors for determining the pressure of fluid flowing in the flow passage positioned both upstream and down stream of the flow restrictor. A pressure regulator responsive to the sensors adjusts the pressure of the fluid either upstream or downstream of the flow restrictor to provide a desired pressure drop across the flow restrictors.
Preferred gas flow controllers have a pair of gas flow passages connected in parallel between the inlet to and outlet from the controller, the flow restrictor is positioned in one of the flow passages, and a valve opens and closes one of the flow passages to gas flow.
Some preferred systems are of modular construction and include a plurality of stacked rectilinear modules. Typically one of the modules defines a by-pass flow passage and includes a control valve, others of the modules define a passage including the flow restrictor, and an ultra-high efficiency gas filter may be mounted in series with the flow restrictor. An electronics module including a memory storing data representing the pressure drop-flow rate characteristics of the flow restrictor receives signals from pressure sensors and outputs a signal for controlling an upstream pressure regulator.
Other objects, features and advantages of the present invention will appear from the following detailed description of preferred embodiments thereof, taken in connection with the drawings.